Method and device for cleaning of a gaseous fluid

ABSTRACT

The present invention relates to a method for cleaning of a gaseous fluid from particles being present in the fluid, the gaseous fluid being ionized, i.e. the particles are subjected to a charging, and that the ionized gaseous fluid is brought to pass through a precipitator unit ( 2 ) including a filter medium. The invention also relates to a device for cleaning of a gaseous fluid. Filters for cleaning of air often require expensive filter medium and complicated arrangements as regards ionization of the air and charging of the filter medium. The present invention is characterized by an extremely simple method where the air is ionized and then the ionized air is brought to pass through a filter medium of polypropylene.

This application is a National Stage of International Application No.PCT/SE98/00818 filed May 5, 1998.

TECHNICAL FIELD OF THE INVENTION

The present invention refers to a method for cleaning of a gaseous fluidfrom particles being present in said fluid, said gaseous fluid beingionised, i.e. the particles are subjected to a charging, and that saidionised gaseous fluid is brought to pass through a precipitator unitincluding a filter medium. The invention also refers to a device forcleaning of a gaseous fluid.

PRIOR ART

From SE-A-9604817-8 a device is previously known in connection with anactive electret filter of the type defined above. Thereby, an electricfield is directed towards the precipitator unit that consists of mediumbeing electrically non-conductive and whose molecules are easilypolarised or oriented by an electric field directed towards theprecipitator unit. By this arrangement an electric field is createdinside the fibre material that the precipitator unit preferably consistsof. The particles that pass through the filter medium, said particlesbeing charged by the ionization unit, are attracted and repelledrespectively by the filter fibres depending on the charging and polarityof the fibres and the particles respectively.

OBJECTS AND FEATURES OF THE INVENTION

The object of the present invention is to define a method and a deviceof the type defined above, the present invention being essentially moresimplified compared to the device that has been described under theheadline “Prior Art”. The objects of the invention is realised by amethod and a device that have been given the features defined in theappending claims.

BRIEF DESCRIPTION OF THE ENCLOSED DRAWINGS

In the drawings:

FIG. 1 is a schematic view of a device according to the invention;

FIG. 2a is a schematic view of a molecule before and after polarisation,said molecule being included in the fibre material of the precipitatorunit;

FIG. 2b is a schematic view showing how the molecules of the filtermedium are oriented;

FIG. 3a is a schematic view showing how an electric field is createdbetween the polarised fibres that are included in the filter medium; and

FIG. 3b shows schematically how the charged particles that pass thefilter are attracted and repelled respectively by the fibres included inthe filter medium.

DETAILED DESCRIPTION OF THE PRINCIPLES OF THE INVENTION AND A SCHEMATICEMBODIMENT OF THE INVENTION

The device shown in FIG. 1 comprises a ionization unit 1 that in theshown embodiment includes a corona tip 3. However, within the scope ofthe invention also other types of ion sources are feasible, e.g. a coalfibre brush or a corona wire. The device also includes a precipitatorunit 2 that below will be described more in detail.

The ionization unit 1 comprises in a known way a high voltage source 4that is connected to the corona tip 3. The ionization unit 1 does notneed to be located adjacent to the precipitator unit 2 but saidionization unit 1 may in principle be located anywhere in the space thatcontains gaseous fluid, said fluid being cleaned by means of the methodand the device according to the present invention.

The precipitator unit 2 consists of a filter of an electricallynon-conductive medium, preferably a fibre filter of polypropylene. It isalso advantageous if the filter medium is a so called fine filter mediumthat in the present application is defined as a filter medium having afibre diameter down to 1 μm and a spacing between the fibres of themagnitude 10 μm. The air velocity through such a fine filter medium isusually 2-10 cm/s.

It is preferred that the fibres, included in the filter medium, are notpre-charged but charged by the particles charged in the ionization unit,i.e. said particles emit their charging to the fibres 7 (see FIGS. 3aand 3 b) in the filter medium when said particles adhere to said filtermedium. This can also be expressed in such a way that the presentinvention preferably does not constitute an electret filter. It is alsoadvantageous if the fibres 7 in other respects are untreated.

The device according to the invention functions in the following way. Inthe space where the ionization unit 1 is located the gaseous fluid,normally air, contains particles that are charged by the ions that theionization unit 1 emits. In this connection it should be repeated thatthe ionization unit 1 may in principle be located anywhere in the spacethat contains the gaseous fluid/air that is to be cleaned.

The particles charged by the ionization unit 1 are in a suitable way,e.g. by means of a fan (not shown), brought to pass through theprecipitator unit 2, this being indicated by the arrows 5. When theseparticles hit the fibres of the filter medium a polarisation of themolecules of said fibres takes place. If it is assumed that a particlehaving a positive charging, i.e. positive corona is used, hits the uppermolecule 6 in FIG. 2a said particle will be polarised in the way shownat the bottom of FIG. 2a. In a corresponding way a fibre 7 that thuscontains a lot of such polarised molecules 6, see FIG. 2b, will have anegative side and a positive side. Charged particles that pass throughthe filter, after chargings have been induced upon the surfaces of thefibres 7, will adhere to said fibres 7, thus a particle having positivecharging will adhere at the left side of the fibre 7 shown in FIG. 2b.

In FIG. 3a it is schematically shown how a fibre 7 upon whose surfacechargings have been induced by the polarised molecules may inducechargings upon the surface of an adjacent fibre 7.

In FIG. 3b is schematically shown how charged particles 8 that passthrough the precipitator unit 2 are attracted and repelled respectivelyby the fibres 7 depending on the charging and polarity of the fibres 7and particles 8 respectively.

The invention is in no way restricted to a positive corona but negativecorona is preferred from certain aspects, a negative corona for instancegenerates less ozone than a positive corona.

What is claimed is:
 1. A device for cleaning of a gaseous fluid fromparticles present in said fluid, comprising: a permanently positive orpermanently negative high voltage source; a filter spaced apart fromsaid source, said filter comprising a fine filter medium composed offibers having a fiber diameter of approximately 1 micron or greater, andan average spacing between said fibers of approximately ten times saidfiber diameter, said fibers being easily polarized when subjected to anelectric charge; means for passing a flow of said fluid past said highvoltage source and through said filter; said filter furthercharacterized by an absence of external charging means other than saidparticles passing through said filter.
 2. A device as defined in claim1, wherein said means for passing said fluid flow past said high voltagesource provides a velocity of said fluid between 2 and 10 cms. persecond.
 3. A device as defined in claim 1, wherein said filter fibershave a diameter of approximately 1 micron.
 4. A device as defined inclaim 1, wherein said filter fibers comprise untreated polypropylene. 5.A device as defined in claim 1, wherein said high voltage sourcecomprises a corona discharge means.
 6. A device as defined in claim 1,wherein said high voltage source comprises a carbon fiber brush.
 7. Amethod for cleaning of a gaseous fluid from particles being present insaid fluid, comprising the steps of: providing a high voltage source anda fine filter medium displaced from said source, said fine filter mediumcomposed of electrically non-conductive fibers having a diameter ofapproximately 1 micron or greater, and an average spacing between saidfibers of approximately ten times said fiber diameter; passing saidfluid past said high voltage source, thereby permanently charging saidparticles either positively or negatively without alternating betweensaid positive or negative charge; passing said charged particles throughsaid filter medium, thereby polarizing said fibers into a positive sideand a negative side thereof without any change in polarity of saidfibers during said cleaning of said fluid; and adhering said chargedparticles to an oppositely-charged side of said fibers, said fibersbeing charged solely by said particles.
 8. A method as defined in claim7, wherein said fibers comprise untreated polypropylene.
 9. A method asdefined in claim 7, wherein said fibers have a diameter of approximately1 micron.
 10. A method as defined in claim 7, wherein the velocity ofsaid fluid is between approximately 2 and 10 cms per second.
 11. Amethod as defined in claim 7, wherein said high voltage discharge meansis selected from one of a corona source or a carbon fiber brush.